For my 4m TX I computed number of turns vs diameter vs length for a couple of air-cored inductors (output filter) using "mini Ring Core Calculator" software.
I picked round diameter values (10mm, 20mm) and adjusted length until I had an integer number of turns. Took adequate lengths of enameled wire and started winding on drill bit. 1 2 3 4 5 turns, done, release finger pression... CLANG! The coil springs back to a wider diameter and fewer turns.
Changed wire with a thinner one, same result plus a more instable coil.
Lesson 1: a coil former is needed. It will hold the wire in place, forever.
But I don't have any 10/20mm former!
Lesson 2: compute the coil according to available coil supports.
I have spare 16/25mm PVC pipe cut-offs. 16mm was behind the door (literally) and large enough. Re-computed coil lengths, cut the pipe to host the wire plus some extra as support.
I also cut a strain relief guide on both sides of each former that keeps the wire in tension.
